1. Field of the Invention
The present invention relates to a novel organic electroluminescense device, and more particularly, to an organic electroluminescence device showing light emission, particularly white light emission, with high efficiency and excellent stability.
2. Description of the Related Arts
An electroluminescence device (hereinafter referred to as EL device) has features in a high self-distinguishability because of its self-emission, and a high impact resistance because it is a completely solid device.
Various EL devices using inorganic and organic compounds are proposed at present and attempts to put them into practice use have been made. Among these devices, organic EL devices permit drastically low voltage to be applied and, therefore, developments of various materials for these devices as well as devices have been undertaken. Furthermore, they are useful for weight reduction of conventionally used devices for exhibition, such as a back light, a display and the like.
Technologies described in the following have heretofore been disclosed on the organic EL devices showing white light emission. However, these technologies have various drawbacks described also in the following.
For example, in a technology disclosed in European Patent Application Laid-Open No. 0390551, a carrier is introduced by tunneling injection making use of accumulation of the cartier at an interface. A threshold voltage exists for emission of white light because of this mechanism. Therefore, gradation cannot be exhibited in this method because light emitted below the threshold voltage is not white. In a technology disclosed in Japanese Patent Application Laid-Open No. 230584/1991, lights of different colors emitted from two kinds of luminescent material are mixed. White light of good quality cannot be obtained because of this mechanism. In a technology disclosed in Japanese Patent Application Laid-Open No. 220390/1990, white light is emitted. However, luminance is 110 cd/m.sup.2 at an applied voltage of 30 V and efficiency of light emission is lower than that expected from the driving voltage. In a technology disclosed in Japanese Patent Application Laid-Open No. 51491/1992, a structure in which light emission is made at face ends is adopted. This is not suitable for applications requiting light emission from the whole surface.